Inverters are well known devices. In general, they are used to convert electrical energy from one form to another. For example, an inverter can use a DC electrical input, such as from a solar array, and provide an AC electrical output. Inverters can also have an AC input. For example, they can be used to output AC electricity at a fixed frequency from a variable frequency AC input. An inverter can be used as a “drive” (also known as a “drive unit”) for controlling operation of motors in a range of different systems. A variety of inverter drive products is available from Control Techniques Ltd, of The Gro, Pool Road, Newtown, Powys, SY16 3BE.
It is also well known to have a system or network wherein multiple devices are connected together in order to communicate with one another. For example a network may comprise a plurality of inverter drive devices. Additionally such a network may comprise a central controller for communicating with each of the other devices on the network.
In order for communications between network nodes, for example communications between two drive devices on the network and/or between a single device and a controller, to be successful, some agreement between the communicating nodes with respect to time is generally required. For example, if a communicated message from one node to another includes an instruction for the receiving node to take a particular action, such as driving a motor at a particular speed at a specified time, it is important that the receiving node has the same understanding of when that specified time will occur as the transmitting node does. If this common understanding is not present, the receiving node will not be in a position to sensibly and accurately action the message in the manner in which the transmitting node intended.
There are many known approaches for networking devices and for attempting to establish an agreement on time between network devices. Known protocols for networking devices include token ring, ARCnet and Ethernet. These protocols and others will be well known to the skilled reader. Each protocol type generally has advantages and disadvantages. For example, certain types of protocol are better suited to networks wherein a single central controller must communicate the same information to a plurality of nodes on the network. Others are better suited to networks in which individual messages are to be communicated to individual respective receiving nodes.